Method for making semiconductor device including superlattice with o18 enriched monolayers

ABSTRACT

A method for making a semiconductor device may include forming a semiconductor layer, and forming a superlattice adjacent the semiconductor layer and including stacked groups of layers. Each group of layers may include stacked base semiconductor monolayers defining a base semiconductor portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The at least one oxygen monolayer of a given group of layers may comprise an atomic percentage of  18 O greater than 10 percent.

TECHNICAL FIELD

The present disclosure generally relates to semiconductor devices and, more particularly, to semiconductor devices with enhanced semiconductor materials and associated methods.

BACKGROUND

Structures and techniques have been proposed to enhance the performance of semiconductor devices, such as by enhancing the mobility of the charge carriers. For example, U.S. Patent Application No. 2003/0057416 to Currie et al. discloses strained material layers of silicon, silicon-germanium, and relaxed silicon and also including impurity-free zones that would otherwise cause performance degradation. The resulting biaxial strain in the upper silicon layer alters the carrier mobilities enabling higher speed and/or lower power devices. Published U.S. Patent Application No. 2003/0034529 to Fitzgerald et al. discloses a CMOS inverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor device including a silicon and carbon layer sandwiched between silicon layers so that the conduction band and valence band of the second silicon layer receive a tensile strain. Electrons having a smaller effective mass, and which have been induced by an electric field applied to the gate electrode, are confined in the second silicon layer, thus, an n-channel MOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice in which a plurality of layers, less than eight monolayers, and containing a fractional or binary or a binary compound semiconductor layer, are alternately and epitaxially grown. The direction of main current flow is perpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short period superlattice with higher mobility achieved by reducing alloy scattering in the superlattice. Along these lines, U.S. Pat. No. 5,683,934 to Candelaria discloses an enhanced mobility MOSFET including a channel layer comprising an alloy of silicon and a second material substitutionally present in the silicon lattice at a percentage that places the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structure comprising two barrier regions and a thin epitaxially grown semiconductor layer sandwiched between the barriers. Each barrier region consists of alternate layers of SiO2/Si with a thickness generally in a range of two to six monolayers. A much thicker section of silicon is sandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also to Tsu and published online Sep. 6, 2000 by Applied Physics and Materials Science & Processing, pp. 391-402 discloses a semiconductor-atomic superlattice (SAS) of silicon and oxygen. The Si/O superlattice is disclosed as useful in a silicon quantum and light-emitting devices. In particular, a green electroluminescence diode structure was constructed and tested. Current flow in the diode structure is vertical, that is, perpendicular to the layers of the SAS. The disclosed SAS may include semiconductor layers separated by adsorbed species such as oxygen atoms, and CO molecules. The silicon growth beyond the adsorbed monolayer of oxygen is described as epitaxial with a fairly low defect density. One SAS structure included a 1.1 nm thick silicon portion that is about eight atomic layers of silicon, and another structure had twice this thickness of silicon. An article to Luo et al. entitled “Chemical Design of Direct-Gap Light-Emitting Silicon” published in Physical Review Letters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the light emitting SAS structures of Tsu.

U.S. Pat. No. 7,105,895 to Wang et al. discloses a barrier building block of thin silicon and oxygen, carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to thereby reduce current flowing vertically through the lattice more than four orders of magnitude. The insulating layer/barrier layer allows for low defect epitaxial silicon to be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al. discloses that principles of Aperiodic Photonic Band-Gap (APBG) structures may be adapted for electronic bandgap engineering. In particular, the application discloses that material parameters, for example, the location of band minima, effective mass, etc., can be tailored to yield new aperiodic materials with desirable band-structure characteristics. Other parameters, such as electrical conductivity, thermal conductivity and dielectric permittivity or magnetic permeability are disclosed as also possible to be designed into the material.

Furthermore, U.S. Pat. No. 6,376,337 to Wang et al. discloses a method for producing an insulating or barrier layer for semiconductor devices which includes depositing a layer of silicon and at least one additional element on the silicon substrate whereby the deposited layer is substantially free of defects such that epitaxial silicon substantially free of defects can be deposited on the deposited layer. Alternatively, a monolayer of one or more elements, preferably comprising oxygen, is absorbed on a silicon substrate. A plurality of insulating layers sandwiched between epitaxial silicon forms a barrier composite.

Despite the existence of such approaches, further enhancements may be desirable for using advanced semiconductor materials and processing techniques to achieve improved performance in semiconductor devices.

SUMMARY

A method for making a semiconductor device may include forming a semiconductor layer, and forming a superlattice adjacent the semiconductor layer and comprising a plurality of stacked groups of layers. Each group of layers may comprise a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The at least one oxygen monolayer of a given group of layers may comprise an atomic percentage of ¹⁸O greater than 10 percent.

By way of example, the at least one oxygen monolayer of the given group of layers may comprise an atomic percentage of ¹⁸O greater than 50 percent, and more particularly greater than 90 percent. The at least one oxygen monolayer of the given group of layers may further comprises ¹⁶O in some embodiments. In an example embodiment, the at least one oxygen monolayer of each group of layers may comprise an atomic percentage of ¹⁸O greater than 10 percent.

In one example configuration, the method may further include forming source and drain regions on the semiconductor layer and defining a channel in the superlattice, and forming a gate above the superlattice. In accordance with another example embodiment, the method may further include forming a metal layer above the superlattice. Furthermore, in some embodiments the superlattice may divide the semiconductor layer into a first region and a second region, with the first region having a same conductivity type and a different dopant concentration than the second region. In accordance with another example implementation, the superlattice may divide the semiconductor layer into a first region and a second region, with the first region having a different conductivity type than the second region. By way of example, the base semiconductor layer may comprise silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged schematic cross-sectional view of a superlattice for use in a semiconductor device in accordance with an example embodiment.

FIG. 2 is a perspective schematic atomic diagram of a portion of the superlattice shown in FIG. 1 .

FIG. 3 is a greatly enlarged schematic cross-sectional view of another embodiment of a superlattice in accordance with an example embodiment.

FIG. 4A is a graph of the calculated band structure from the gamma point (G) for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in FIGS. 1-2 .

FIG. 4B is a graph of the calculated band structure from the Z point for both bulk silicon as in the prior art, and for the 4/1 Si/O superlattice as shown in FIGS. 1-2 .

FIG. 4C is a graph of the calculated band structure from both the gamma and Z points for both bulk silicon as in the prior art, and for the 5/1/3/1 Si/O superlattice as shown in FIG. 3 .

FIG. 5 is a schematic cross-sectional view of a semiconductor device including a superlattice with an enriched ¹⁸O monolayer in accordance with an example embodiment.

FIG. 6 is a schematic cross-sectional view of another semiconductor device including a superlattice with a plurality of enriched ¹⁸O monolayers and dividing a semiconductor layer into regions of different conductivity types.

FIG. 7 is a graph of measured oxygen concentration vs depth for a test semiconductor device including typical ¹⁶O monolayers and ¹⁸O enhanced monolayers.

FIG. 8 is a table showing ¹⁸O and ¹⁶O dose loss for the implementation of FIG. 7 .

FIG. 9 is a graph of ¹⁸O and ¹⁶O dose loss vs. anneal temperature for the implementation of FIG. 7 .

FIG. 10 is a graph of ¹⁸O and ¹⁶O dose loss percentage vs. anneal temperature for the implementation of FIG. 7 .

FIG. 11 is a schematic cross-sectional view of a semiconductor device including a superlattice channel with one or more enriched ¹⁸O monolayers.

FIG. 12 is a schematic cross-sectional view of a semiconductor device including a superlattice with one or more enriched ¹⁸O monolayers and dividing a semiconductor layer into regions having a same conductivity type and different dopant concentrations.

FIG. 13 is a schematic cross-sectional view of a semiconductor device including a superlattice with one or more enriched ¹⁸O monolayers and including a metal contact layer above the superlattice.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which the example embodiments are shown. The embodiments may, however, be implemented in many different forms and should not be construed as limited to the specific examples set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in different embodiments.

Generally speaking, the present disclosure relates to the formation of semiconductor devices utilizing an enhanced semiconductor superlattice. The enhanced semiconductor superlattice may also be referred to as an “MST” layer/film or “MST technology” in this disclosure.

More particularly, the MST technology relates to advanced semiconductor materials such as the superlattice 25 described further below. Applicant theorizes, without wishing to be bound thereto, that certain superlattices as described herein reduce the effective mass of charge carriers and that this thereby leads to higher charge carrier mobility. Effective mass is described with various definitions in the literature. As a measure of the improvement in effective mass Applicant's use a “conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹ for electrons and holes respectively, defined as:

${M_{e,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}{\int\limits_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}{\sum\limits_{E > E_{F}}{\int\limits_{B.Z.}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}d^{3}k}}}$

for electrons and:

${M_{h,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{- {\sum\limits_{E < E_{F}}{\int\limits_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}d^{3}k}}}}{\sum\limits_{E < E_{F}}{\int\limits_{B.Z.}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}} \right)d^{3}k}}}$

for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermi energy, T is the temperature, E(k,n) is the energy of an electron in the state corresponding to wave vector k and the nth energy band, the indices i and j refer to Cartesian coordinates x, y and z, the integrals are taken over the Brillouin zone (B.Z.), and the summations are taken over bands with energies above and below the Fermi energy for electrons and holes respectively.

Applicant's definition of the conductivity reciprocal effective mass tensor is such that a tensorial component of the conductivity of the material is greater for greater values of the corresponding component of the conductivity reciprocal effective mass tensor. Again, Applicant theorizes without wishing to be bound thereto that the superlattices described herein set the values of the conductivity reciprocal effective mass tensor so as to enhance the conductive properties of the material, such as typically for a preferred direction of charge carrier transport. The inverse of the appropriate tensor element is referred to as the conductivity effective mass. In other words, to characterize semiconductor material structures, the conductivity effective mass for electrons/holes as described above and calculated in the direction of intended carrier transport is used to distinguish improved materials.

Applicant has identified improved materials or structures for use in semiconductor devices. More specifically, Applicant has identified materials or structures having energy band structures for which the appropriate conductivity effective masses for electrons and/or holes are substantially less than the corresponding values for silicon. In addition to the enhanced mobility characteristics of these structures, they may also be formed or used in such a manner that they provide piezoelectric, pyroelectric, and/or ferroelectric properties that are advantageous for use in a variety of different types of devices, as will be discussed further below.

Referring now to FIGS. 1 and 2 , the materials or structures are in the form of a superlattice 25 whose structure is controlled at the atomic or molecular level and may be formed using known techniques of atomic or molecular layer deposition. The superlattice 25 includes a plurality of layer groups 45 a-45 n arranged in stacked relation, as perhaps best understood with specific reference to the schematic cross-sectional view of FIG. 1 .

Each group of layers 45 a-45 n of the superlattice 25 illustratively includes a plurality of stacked base semiconductor monolayers 46 defining a respective base semiconductor portion 46 a-46 n and an energy band-modifying layer 50 thereon. The energy band-modifying layers 50 are indicated by stippling in FIG. 1 for clarity of illustration.

The energy band-modifying layer 50 illustratively includes one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions. By “constrained within a crystal lattice of adjacent base semiconductor portions” it is meant that at least some semiconductor atoms from opposing base semiconductor portions 46 a-46 n are chemically bound together through the non-semiconductor monolayer 50 therebetween, as seen in FIG. 2 . Generally speaking, this configuration is made possible by controlling the amount of non-semiconductor material that is deposited on semiconductor portions 46 a-46 n through atomic layer deposition techniques so that not all (i.e., less than full or 100% coverage) of the available semiconductor bonding sites are populated with bonds to non-semiconductor atoms, as will be discussed further below. Thus, as further monolayers 46 of semiconductor material are deposited on or over a non-semiconductor monolayer 50, the newly deposited semiconductor atoms will populate the remaining vacant bonding sites of the semiconductor atoms below the non-semiconductor monolayer.

In other embodiments, more than one such non-semiconductor monolayer may be possible. It should be noted that reference herein to a non-semiconductor or semiconductor monolayer means that the material used for the monolayer would be a non-semiconductor or semiconductor if formed in bulk. That is, a single monolayer of a material, such as silicon, may not necessarily exhibit the same properties that it would if formed in bulk or in a relatively thick layer, as will be appreciated by those skilled in the art.

Applicant theorizes without wishing to be bound thereto that energy band-modifying layers 50 and adjacent base semiconductor portions 46 a-46 n cause the superlattice 25 to have a lower appropriate conductivity effective mass for the charge carriers in the parallel layer direction than would otherwise be present. Considered another way, this parallel direction is orthogonal to the stacking direction. The band modifying layers 50 may also cause the superlattice 25 to have a common energy band structure, while also advantageously functioning as an insulator between layers or regions vertically above and below the superlattice.

Moreover, this superlattice structure may also advantageously act as a barrier to dopant and/or material diffusion between layers vertically above and below the superlattice 25. These properties may thus advantageously allow the superlattice 25 to provide an interface for high-K dielectrics which not only reduces diffusion of the high-K material into the channel region, but which may also advantageously reduce unwanted scattering effects and improve device mobility, as will be appreciated by those skilled in the art.

It is also theorized that semiconductor devices including the superlattice 25 may enjoy a higher charge carrier mobility based upon the lower conductivity effective mass than would otherwise be present. In some embodiments, and as a result of the band engineering achieved by the present invention, the superlattice 25 may further have a substantially direct energy bandgap that may be particularly advantageous for opto-electronic devices, for example.

The superlattice 25 also illustratively includes a cap layer 52 on an upper layer group 45 n. The cap layer 52 may comprise a plurality of base semiconductor monolayers 46. By way of example, the cap layer 52 may have between 1 to 100 monolayers 46 of the base semiconductor, and, more preferably between 10 to 50 monolayers. However, in some applications the cap layer 52 may be omitted, or thicknesses greater than 100 monolayers may be used.

Each base semiconductor portion 46 a-46 n may comprise a base semiconductor selected from the group consisting of Group IV semiconductors, Group III-V semiconductors, and Group II-VI semiconductors. Of course, the term Group IV semiconductors also includes Group IV-IV semiconductors, as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductor selected from the group consisting of oxygen, nitrogen, fluorine, carbon and carbon-oxygen, for example. The non-semiconductor is also desirably thermally stable through deposition of a next layer to thereby facilitate manufacturing. In other embodiments, the non-semiconductor may be another inorganic or organic element or compound that is compatible with the given semiconductor processing as will be appreciated by those skilled in the art. More particularly, the base semiconductor may comprise at least one of silicon and germanium, for example.

It should be noted that the term monolayer is meant to include a single atomic layer and also a single molecular layer. It is also noted that the energy band-modifying layer 50 provided by a single monolayer is also meant to include a monolayer wherein not all of the possible sites are occupied (i.e., there is less than full or 100% coverage). For example, with particular reference to the atomic diagram of FIG. 2 , a 4/1 repeating structure is illustrated for silicon as the base semiconductor material, and oxygen as the energy band-modifying material. Only half of the possible sites for oxygen are occupied in the illustrated example.

In other embodiments and/or with different materials this one-half occupation would not necessarily be the case as will be appreciated by those skilled in the art. Indeed, it can be seen even in this schematic diagram, that individual atoms of oxygen in a given monolayer are not precisely aligned along a flat plane as will also be appreciated by those of skill in the art of atomic deposition. By way of example, a preferred occupation range is from about one-eighth to one-half of the possible oxygen sites being full, although other numbers may be used in certain embodiments.

Silicon and oxygen are currently widely used in conventional semiconductor processing, and, hence, manufacturers will be readily able to use these materials as described herein. Atomic or monolayer deposition is also now widely used. Accordingly, semiconductor devices incorporating the superlattice 25 in accordance with the invention may be readily adopted and implemented, as will be appreciated by those skilled in the art.

It is theorized without Applicant wishing to be bound thereto that for a superlattice, such as the Si/O superlattice, for example, that the number of silicon monolayers should desirably be seven or less so that the energy band of the superlattice is common or relatively uniform throughout to achieve the desired advantages. The 4/1 repeating structure shown in FIGS. 1 and 2 , for Si/O has been modeled to indicate an enhanced mobility for electrons and holes in the X direction. For example, the calculated conductivity effective mass for electrons (isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice in the X direction it is 0.12 resulting in a ratio of 0.46. Similarly, the calculation for holes yields values of 0.36 for bulk silicon and 0.16 for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired in certain semiconductor devices, other devices may benefit from a more uniform increase in mobility in any direction parallel to the groups of layers. It may also be beneficial to have an increased mobility for both electrons and holes, or just one of these types of charge carriers as will be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of the superlattice 25 may be less than two-thirds the conductivity effective mass than would otherwise occur, and this applies for both electrons and holes. Of course, the superlattice 25 may further comprise at least one type of conductivity dopant therein, as will also be appreciated by those skilled in the art.

Indeed, referring now additionally to FIG. 3 , another embodiment of a superlattice 25′ in accordance with the invention having different properties is now described. In this embodiment, a repeating pattern of 3/1/5/1 is illustrated. More particularly, the lowest base semiconductor portion 46 a′ has three monolayers, and the second lowest base semiconductor portion 46 b′ has five monolayers. This pattern repeats throughout the superlattice 25′. The energy band-modifying layers 50′ may each include a single monolayer. For such a superlattice 25′ including Si/O, the enhancement of charge carrier mobility is independent of orientation in the plane of the layers. Those other elements of FIG. 3 not specifically mentioned are similar to those discussed above with reference to FIG. 1 and need no further discussion herein.

In some device embodiments, all of the base semiconductor portions of a superlattice may be a same number of monolayers thick. In other embodiments, at least some of the base semiconductor portions may be a different number of monolayers thick. In still other embodiments, all of the base semiconductor portions may be a different number of monolayers thick.

In FIGS. 4A-4C, band structures calculated using Density Functional Theory (DFT) are presented. It is well known in the art that DFT underestimates the absolute value of the bandgap. Hence all bands above the gap may be shifted by an appropriate “scissors correction.” However, the shape of the band is known to be much more reliable. The vertical energy axes should be interpreted in this light.

FIG. 4A shows the calculated band structure from the gamma point (G) for both bulk silicon (represented by continuous lines) and for the 4/1 Si/O superlattice 25 shown in FIG. 1 (represented by dotted lines). The directions refer to the unit cell of the 4/1 Si/O structure and not to the conventional unit cell of Si, although the (001) direction in the figure does correspond to the (001) direction of the conventional unit cell of Si, and, hence, shows the expected location of the Si conduction band minimum. The (100) and (010) directions in the figure correspond to the (110) and (−110) directions of the conventional Si unit cell. Those skilled in the art will appreciate that the bands of Si on the figure are folded to represent them on the appropriate reciprocal lattice directions for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/O structure is located at the gamma point in contrast to bulk silicon (Si), whereas the valence band minimum occurs at the edge of the Brillouin zone in the (001) direction which we refer to as the Z point. One may also note the greater curvature of the conduction band minimum for the 4/1 Si/O structure compared to the curvature of the conduction band minimum for Si owing to the band splitting due to the perturbation introduced by the additional oxygen layer.

FIG. 4B shows the calculated band structure from the Z point for both bulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25 (dotted lines). This figure illustrates the enhanced curvature of the valence band in the (100) direction.

FIG. 4C shows the calculated band structure from both the gamma and Z point for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/O structure of the superlattice 25′ of FIG. 3 (dotted lines). Due to the symmetry of the 5/1/3/1 Si/O structure, the calculated band structures in the (100) and (010) directions are equivalent. Thus, the conductivity effective mass and mobility are expected to be isotropic in the plane parallel to the layers, i.e. perpendicular to the (001) stacking direction. Note that in the 5/1/3/1 Si/O example the conduction band minimum and the valence band maximum are both at or close to the Z point.

Although increased curvature is an indication of reduced effective mass, the appropriate comparison and discrimination may be made via the conductivity reciprocal effective mass tensor calculation. This leads Applicant to further theorize that the 5/1/3/1 superlattice 25′ should be substantially direct bandgap. As will be understood by those skilled in the art, the appropriate matrix element for optical transition is another indicator of the distinction between direct and indirect bandgap behavior.

Turning now to FIG. 5 , in some embodiments the above-described superlattice films 25 may be fabricated with one or more oxygen monolayers 50 which have an increased or enhanced amount of ¹⁸O. In a typical fabrication process, the approximate concentration of stable oxygen isotopes present in the gas flow used for oxygen deposition may be as follows:

Isotope Mass (Da) [%] ¹⁶O 15.99492 99.757 ¹⁷O 16.99913 0.038 ¹⁸O 17.99916 0.205 In the semiconductor device 120 shown in FIG. 5 , a superlattice 125 is formed adjacent a semiconductor layer 121 (e.g., substrate) which includes two groups of monolayers 145 a, 145 b, each including a base semiconductor portion 146 a, 146 b with four semiconductor (e.g., silicon) monolayers 146, and a respective oxygen monolayer 150 a, 150 b. However, it should be noted that other base semiconductor portion thicknesses may be used in different embodiments, e.g., up to twenty-five monolayers 146 or even fifty monolayers (or more) in some implementations. While the oxygen monolayer 150 b is fabricated using a typical gas flow, the oxygen monolayer 150 a is fabricated using a gas flow having an enhanced or increased amount of ¹⁸O to thereby provide an ¹⁸O enriched monolayer. By way of example, the monolayer 150 a may comprise an atomic percentage of ¹⁸O greater than ten percent. That is, the number of ¹⁸O atoms present in the monolayer 150 a may constitute 10% or more of the total oxygen atoms in this monolayer(s). In other example embodiments, the atomic percentage of ¹⁸O atoms in the monolayer 150 a may be greater than fifty percent of the total oxygen atoms, and more particularly greater than ninety percent. In any case, the ¹⁸O enriched monolayer 150 a may also include some portion of ¹⁶O.

Referring additionally to the semiconductor device 120′ of FIG. 6 , in some implementations more than one ¹⁸O enriched monolayer 150 a′ may be used. Here, each of the two groups of layers 145 a′, 145 b′ has a respective ¹⁸O enriched monolayer 150 a′. Other superlattice layer configurations may also be used in different embodiments.

The use of one or more ¹⁸O enriched monolayers 150 a in an MST layer may be advantageous in view of the kinetic isotope effect of interstitial oxygen within the semiconductor (e.g., silicon) lattice of the base semiconductor portions 146 a, 146 b. More particularly, free oxygen atoms in silicon are relatively highly mobile, which may lead to unwanted diffusion via an interstitial mechanism. Diffusion of oxygen is thermally activated, and is therefore susceptible to occur in subsequent thermal processing steps (e.g., gate formation, etc.) after the superlattice 125 formation. Because ¹⁸O is chemically equivalent to ¹⁶O in terms of its nuclear spin (both are 0), it is well suited for use in the above-described superlattice structures where oxygen monolayers with typical ¹⁶O concentrations would otherwise be used. However, as a result of the kinetic isotope effect, activation energy for a lighter isotope is less than for a heavier isotope. In the present example, ¹⁶O is a lighter isotope than ¹⁸O, meaning that ¹⁸O will have a higher activation energy than ¹⁶O. Thus, activated processes for ¹⁸O are accordingly slower, meaning that ¹⁸O will diffuse more slowly than ¹⁶O. As a result, and as theorized by Applicant without wishing to be bound thereto, ¹⁸O enriched monolayers 150 a will experience less diffusion/oxygen loss during the above-noted thermal processing, for example.

The foregoing will be further understood with reference to the graph 170 of FIG. 7 , table 180 of FIG. 8 , and graphs 190, 195 of FIGS. 9 and 10 representing test results from a fabricated device including four ¹⁶O monolayers and four ¹⁸O enriched monolayers. The test device was fabricated using an etch-back procedure (referred to as “MEGA” in the figures) during fabrication that is described further in U.S. Pat Nos. 10,566,191 and 10,811,498 to Weeks et al., which are assigned to the present Applicant and hereby incorporated herein in their entireties by reference. The ¹⁸O concentration is represented by the plot line 171, while the ¹⁶O concentration is represented by the plotline 172. It may be seen that the location of the oxygen monolayer 150 a occurs between 20 and 30 nm from the surface and has an ¹⁸O concentration in a range of 1×10²¹ atoms/cm³. The corresponding measurements for the test film are shown in the table 180 of FIG. 8 , the corresponding dose loss vs. anneal temperature is shown in the graph 190, and the corresponding dose loss percentage vs. anneal temperature is shown in the graph 195 of FIG. 10 .

Numerous types of semiconductor structures may be fabricated with, and benefit from, the above-described ¹⁸O enhanced superlattices 120 or 120′. One such device is a planar MOSFET 220 now described with reference to FIG. 11 . The illustrated MOSFET 220 includes a substrate 221, source/drain regions 222, 223, source/drain extensions 226, 227, and a channel region therebetween provided by an ¹⁸O enhanced superlattice 225. The channel may be formed partially or completely within the superlattice 225. Source/drain silicide layers 230, 231 and source/drain contacts 232, 233 overlie the source/drain regions as will be appreciated by those skilled in the art. Regions indicated by dashed lines 234 a, 234 b are optional vestigial portions formed originally with the superlattice 225, but thereafter heavily doped. In other embodiments, these vestigial superlattice regions 234 a, 234 b may not be present as will also be appreciated by those skilled in the art. A gate 235 illustratively includes a gate insulating layer 237 adjacent the channel provided by the superlattice 225, and a gate electrode layer 236 on the gate insulating layer. Sidewall spacers 240, 241 are also provided in the illustrated MOSFET 220.

Referring additionally to FIG. 12 , in accordance with another example of a device in which an ¹⁸O enriched superlattice 325 may be incorporated is a semiconductor device 300, in which the superlattice is used as a dopant diffusion blocking superlattice to advantageously increase surface dopant concentration to allow a higher N_(D) (active dopant concentration at metal/semiconductor interface) during in-situ doped epitaxial processing by preventing diffusion into a channel region 330 of the device. More particularly, the device 100 illustratively includes a semiconductor layer or substrate 301, and spaced apart source and drain regions 302, 303 formed in the semiconductor layer with the channel region 330 extending therebetween. The dopant diffusion blocking superlattice 325 illustratively extends through the source region 302 to divide the source region into a lower source region 304 and an upper source region 305, and also extends through the drain region 303 to divide the drain region into a lower drain region 306 and an upper drain region 307.

The dopant diffusion blocking superlattice 325 may also conceptually be considered as a source dopant blocking superlattice within the source region 302, a drain dopant blocking superlattice within the drain region 303, and a body dopant blocking superlattice beneath the channel 330, although in this configuration all three of these are provided by a single blanket deposition of the MST material across the substrate 301 as a continuous film. The semiconductor material above the dopant blocking superlattice 325 in which the upper source/drain regions 305, 307 and channel region 330 are defined may be epitaxially grown on the dopant blocking superlattice 325 either as a thick superlattice cap layer or bulk semiconductor layer, for example. In the illustrated example, the upper source/drain regions 305, 307 may each be level with an upper surface of this semiconductor layer (i.e., they are implanted within this layer).

As such, the upper source/drain regions 305, 307 may advantageously have a same conductivity as the lower source/drain regions 304, 306, yet with a higher dopant concentration. In the illustrated example, the upper source/drain regions 305, 307 and the lower source/drain regions 304, 306 are N-type for a N-channel device, but these regions may also be P-type for a P-channel device as well. Surface dopant may be introduced by ion implantation, for example. Yet, the dopant diffusion is reduced by the MST film material of the diffusion blocking superlattice 325 because it traps point defects/interstitials introduced by ion implantation which mediate dopant diffusion.

The semiconductor device 300 further illustratively includes a gate 308 on the channel region 330. The gate illustratively includes a gate insulating layer 309 gate electrode 310. Sidewall spacers 311 are also provided in the illustrated example. Further details regarding the device 300, as well as other similar structures in which an ¹⁸O enriched superlattice may be used, are set forth in U.S. Pat. No. 10,818,755 to Takeuchi et al., which is assigned to the present Applicant and hereby incorporated herein in its entirety by reference.

Turning to FIG. 13 , another example embodiment of a semiconductor device 400 in which an ¹⁸O enriched superlattice may be used is now described. More particularly, in the illustrated example both source and drain dopant diffusion blocking superlattices 425 s, 425 d advantageously provide for Schottky barrier height modulation via hetero-epitaxial film integration. More particularly, the lower source and drain regions 404, 406 include a different material than the upper source and drain regions 405, 407. In this example, the lower source and drain regions 404, 406 are silicon, and the upper source and drain regions 405, 407 are SiGeC, although different materials may be used in different embodiments. Lower metal layers (Ti) 442, 443 are formed on the upper source and drain regions (SiGeC layers) 405, 407. Upper metal layers (Co) 444, 445 are formed on the lower metal layers 442, 443, respectively. Because the MST material is effective in integrating hetero-epitaxial semiconductor material, incorporation of C(1-2%) to Si or SiGe on Si may induce a positive conduction band offset. More particularly, this is a SiGeC/MST/n+ Si structure that is effective for reducing Schottky barrier height. Further details regarding the device 400 are set forth in the above-noted '755 patent.

One skilled in the art, however, will appreciate that the materials and techniques identified herein may be used in many different types of semiconductor devices, such as discrete devices and/or integrated circuits. Referring again to FIG. 6 , in the context of dopant blocking applications, the ¹⁸O enriched superlattice 125′ divides the substrate 121′ and the cap layer 52′, but the substrate has a different conductivity type (P) than the cap layer (N) to thereby define a PN junction. In other example embodiments, the PN junction may be lateral, as opposed to vertically oriented as shown in the example of FIG. 6 . Further PN junction applications in which ¹⁸O enriched superlattice may be used are set forth in U.S. Pat. No. 7,227,174 to Mears et al., which is assigned to the present Applicant and hereby incorporated herein in its entirety by reference. It should also be noted that in some embodiments, ¹⁸O enriched monolayers may also be incorporated in superlattices and associated applications such as those described in co-pending application Ser. Nos. 17/236,289 and 17/236,329 filed Apr. 21, 2021, which are hereby incorporated herein in their entireties by reference.

Applicant theorizes, without wishing to be bound thereto, that an ¹⁸O source can be used interchangeably with traditional ¹⁶O sources to fabricate the above-described semiconductor superlattices. Moreover, Applicant has found that similar ¹⁸O flow rates yield similar oxygen dosages to those of ¹⁶O. Furthermore, the semiconductor monolayer growth and etch rates are also similar between ¹⁶O and ¹⁸O sources. Phenomenological study/observations have revealed that ¹⁶O incorporation in the ¹⁸O superlattice layers is affected by the above-described MEGA etch. More particularly, with respect to the test device represented in FIG. 7 , implementations of the device without the MEGA etch showed the first ¹⁶O to be lower than the other three ¹⁶O peaks in the same stack, whereas adding a MEGA etch before the first oxygen dosing cycle resulted in a superlattice stack with all four ¹⁶O peaks being of the same concentration. By way of example, ¹⁸O dose retention may be 30% better (or more) than ¹⁶O dose retention.

A related method for making a semiconductor device 120 may include forming a semiconductor layer 121, and forming a superlattice 125 adjacent the semiconductor layer and including stacked groups of layers 145 a, 145 b. Each group of layers 145 a, 145 b may include stacked base semiconductor monolayers 146 defining a base semiconductor portion 146 a, 146 b, and at least one oxygen monolayer 150 a constrained within a crystal lattice of adjacent base semiconductor portions. The at least one oxygen monolayer 150 a may comprise an atomic percentage of ¹⁸O greater than 10 percent, as discussed further above.

In accordance with the example of FIG. 11 , further method aspects may include forming source and drain regions 222, 223 on the semiconductor layer 221 and defining a channel in the superlattice 225, and forming a gate 235 above the superlattice. In accordance with the example of FIG. 13 , further method aspects may include forming a metal layer 442/444 and/or 443/445 above the superlattice 425 s, 425 d, as discussed further above.

Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims. 

That which is claimed is:
 1. A method for making a semiconductor device comprising: forming a semiconductor layer; and forming a superlattice adjacent the semiconductor layer and comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base semiconductor portions; the at least one oxygen monolayer of a given group of layers comprising an atomic percentage of ¹⁸O greater than 10 percent.
 2. The method of claim 1 wherein the at least one oxygen monolayer of the given group of layers comprises an atomic percentage of ¹⁸O greater than 50 percent.
 3. The method of claim 1 wherein the at least one oxygen monolayer of the given group of layers comprises an atomic percentage of ¹⁸O greater than 90 percent.
 4. The method of claim 1 wherein the at least one oxygen monolayer of the given group of layers further comprises ¹⁶O.
 5. The method of claim 1 wherein the at least one oxygen monolayer of each group of layers comprises an atomic percentage of ¹⁸O greater than 10 percent.
 6. The method of claim 1 further comprising forming source and drain regions on the semiconductor layer and defining a channel in the superlattice, and forming a gate above the superlattice.
 7. The method of claim 1 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a same conductivity type and a different dopant concentration than the second region.
 8. The method of claim 1 further comprising forming a metal layer above the superlattice.
 9. The method of claim 1 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a different conductivity type than the second region.
 10. The method of claim 1 wherein the base semiconductor layer comprises silicon.
 11. A method for making a semiconductor device comprising: forming a semiconductor layer; and forming a superlattice adjacent the semiconductor layer and comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base silicon portions; the at least one oxygen monolayer of a given group of layers comprising an atomic percentage of ¹⁸O greater than 50 percent.
 12. The method of claim 11 wherein the at least one oxygen monolayer of the given group of layers comprises an atomic percentage of ¹⁸O greater than 90 percent.
 13. The method of claim 11 wherein the at least one oxygen monolayer of the given group of layers comprises ¹⁶O.
 14. The method of claim 11 wherein the at least one oxygen monolayer of each group of layers within the superlattice comprises an atomic percentage of ¹⁸O greater than 50 percent.
 15. The method of claim 11 further comprising forming source and drain regions on the semiconductor layer and defining a channel in the superlattice, and forming a gate above the superlattice.
 16. The method of claim 11 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a same conductivity type and a different dopant concentration than the second region.
 17. The method of claim 11 further comprising forming a metal layer above the superlattice.
 18. The method of claim 11 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a different conductivity type than the second region.
 19. A method for making a semiconductor device comprising: forming a semiconductor layer; and forming a superlattice adjacent the semiconductor layer and comprising a plurality of stacked groups of layers, each group of layers comprising a plurality of stacked base silicon monolayers defining a base silicon portion, and at least one oxygen monolayer constrained within a crystal lattice of adjacent base silicon portions; each at least one oxygen monolayer constrained within the crystal lattice of adjacent base silicon portions comprising an atomic percentage of ¹⁸O greater than 90 percent.
 20. The method of claim 19 wherein each at least one oxygen monolayer comprises ¹⁶O.
 21. The method of claim 19 further comprising forming source and drain regions on the semiconductor layer and defining a channel in the superlattice, and forming a gate above the superlattice.
 22. The method of claim 19 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a same conductivity type and a different dopant concentration than the second region.
 23. The method of claim 19 further comprising forming a metal layer above the superlattice.
 24. The method of claim 19 wherein the superlattice divides the semiconductor layer into a first region and a second region, with the first region having a different conductivity type than the second region. 